1. Field of the Invention
This invention relates to a cleaner and surface conditioner for formed metal surfaces, and particularly, to such a lubricant and surface conditioner which improves the mobility of aluminum cans without adversely affecting the adhesion of paints or lacquers applied thereto, and also enables lowering the dryoff oven temperature required for drying said surfaces. Still more particularly, this invention relates to a combination of cleaning and such surface conditioning which minimizes the formation of sludge or other undesirable phase separation during the process of surface conditioning when the surface conditioner contains metallic elements as part of its chemical composition.
2. Discussion of Related Art
Aluminum cans are commonly used as containers for a wide variety of products. After their manufacture, the aluminum cans are typically washed with acidic cleaners to remove aluminum fines and other contaminants therefrom. Recently, environmental considerations and the possibility that residues remaining on the cans following acidic cleaning could influence the flavor of beverages packaged in the cans has led to an interest in alkaline cleaning to remove such fines and contaminants. However, the treatment of aluminum cans generally results in differential rates of metal surface etch on the outside versus on the inside of the cans. For example, optimum conditions required to attain an aluminum fines-free surface on the inside of the cans usually leads to can mobility problems on conveyors because of the increased roughness on the outside can surface.
These aluminum can mobility problems are particularly apparent when it is attempted to convey the cans through single filers and to printers. Thus, a need has arisen in the aluminum can manufacturing industry to modify the coefficient of static friction on the outside and inside surfaces of the cans to improve their mobility without adversely affecting the adhesion of paints or lacquers applied thereto. The reason for improving the mobility of aluminum cans is the general trend in this manufacturing industry to increase production without additional capital investments in building new plants. The increased production demand is requiring can manufacturers to increase their line and printer speeds to produce more cans per unit of time. For example, the maximum speed at which aluminum cans, in the absence of any treatment to reduce their coefficient of surface friction, may be passed through a printing station typically is on the average of about 1150 cans per minute, whereas it is desired that such rate be increased to about 1800 to 2000 cans per minute or even higher.
However, aluminum cans thoroughly cleaned by either acid or alkaline cleaners are, in general, characterized by high surface roughness and thus have a high coefficient of static friction. This property hinders the flow of cans through single filers and printers when attempting to increase their line speed. As a result, printer misfeeding problems, frequent jammings, down time, and loss of production occur in addition to high rates of can spoilage.
Another consideration in modifying the surface properties of aluminum cans is the concern that such modification may interfere with or adversely affect the ability of the can to be printed when passed to a printing or labeling station. For example, after cleaning the cans, labels may be printed on their outside surface, and lacquers may be sprayed on their inside surface. In such a case, the adhesion of the paints and lacquers is of major concern.
In addition, the current trend in the can manufacturing industry is directed toward using thinner gauges of aluminum metal stock. The down-gauging of aluminum can metal stock has caused a production problem in that, after washing, the cans require a lower drying oven temperature in order to pass the column strength pressure quality control test. However, lowering the drying oven temperature resulted in the cans not being dry enough when they reached the printing station, and caused label ink smears and a higher rate of can rejects.
Thus, it would be desirable to provide a means of improving the mobility of aluminum cans through single filers and printers to increase production, reduce line jammings, minimize down time, reduce can spoilage, improve ink laydown, and enable lowering the drying oven temperature of washed cans. Accordingly, it is an object of this invention to provide such means of improving the mobility of aluminum cans and to overcome the afore-noted problems.
In the most widely used current commercial practice, at least for large scale operations, aluminum cans are typically subjected to a succession of six cleaning and rinsing operations as described in Table A below. (Contact with ambient temperature tap water before any of the stages in Table A is sometimes used also; when used, this stage is often called a "vestibule" to the numbered stages.)
TABLE A ______________________________________ STAGE NUMBER ACTION ON SURFACE DURING STAGE ______________________________________ 1 Aqueous Acid Precleaning 2 Aqueous Acid and Surfactant Cleaning 3 Tap Water Rinse 4 Mild Acid Postcleaning, Conversion Coating, or Tap Water Rinse 5 Tap Water Rinse 6 Deionized ("DI") Water Rinse ______________________________________
It is currently possible to produce a can which is satisfactorily mobile and to which subsequently applied inks and/or lacquers have adequate adhesion by using suitable surfactants either in Stage 4 or Stage 6 as noted above. Preferred treatments for use in Stage 4 as described above have been developed and are described in U.S. Pat. Nos. 5,030,323 and 5,064,500. With these treatments, a metallic element (not necessarily or even usually in elemental form) is incorporated into the lubricant and surface conditioning layer formed.
Experience with prolonged practical use of lubricant and surface conditioner forming treatments that incorporate metal into the surface conditioner layer formed has revealed that they are susceptible to the development of at least one separate impurity phase, commonly called "sludge" or some similar term. The sludge is usually sticky, so that small particles of it easily adhere to the containers being treated, and if they do so can cause an undesirable phenomenon called "metal exposure", a failure of the subsequently applied interior sanitary lacquer to completely isolate the beverage product contained in the aluminum can from contact with the metal can body. Therefore, if a sufficient amount of sludge forms, it must be removed before continuing with can conditioning. Because of the tackiness of the sludge, it is difficult to remove satisfactorily, so that minimizing and, if possible, preventing formation of the sludge is one of the objects of this invention.